For approximately 30 years, high resolution focused ion beams (FIBs) have proven useful for a variety of tasks such as microscopy, lithography, micromachining (ion milling and material deposition), and dopant implantation. Over the years, a number of ion sources have been developed for focused ion beam applications, including gas-phase field ionization, plasma, and liquid metals. Of all the sources developed to date, the liquid metal ion source (LMIS) has proven the most useful and is in the most widespread use today. The usefulness of the liquid metal ion source stems fundamentally from its very high brightness which allows the production of focused ion beams with spot sizes on the order of 10 nm while maintaining currents in the range of 1 pA to 10 pA. These characteristics give focused ion beams the necessary resolution and ion currents to perform a range of state of the art nanotechnology tasks.
Despite their widespread use, existing ion sources possess limitations that impede progress toward broader applications and higher resolution. Because of the need to wet a tungsten tip with a liquid metal, the number of different ionic species that can be implemented in a liquid metal ion source is somewhat limited. Ga is by far the predominant element used, though other species, including Au, Al, Be, and Cs, have been demonstrated. The liquid metal ion source also suffers from an extremely large energy spread, more than several eV, which is generally considered attributable to space charge effects occurring near the very small emission area on the surface of the emitter. This energy broadening leads to chromatic aberration in the focusing optics that form the focused ion beam, thereby limiting the achievable resolution and forcing a trade off between beam current and resolution. Gas phase field ionization sources address some of these problems in that they can operate with light elements and have a narrower energy spread, on the order of 1 eV, but the current is significantly less, they do not work with heavy elements, and they are more complicated to operate. Plasma sources also overcome some of the problems of the liquid metal ion source, but their brightness is orders of magnitude less than the other two sources. A further practical issue relevant to liquid metal and gas phase sources is that the nanometer scale effective source size, required for the existing sources to have high brightness, translates into a very acute sensitivity to source positional stability, which becomes an issue in the construction of a focused ion beam system.
Accordingly, a need exists for an improved system and strategy for generating ions, and particularly a focused ion beam that is suitable for a wide array of applications such as for example, site analyses, material deposition or implantation, ablation of materials, ion microscopy, secondary ion mass spectroscopy (SIMS), and ion nano-machining.
U.S. Pat. Pub. No. 2008/0296483 for “Magneto-optical Trap Ion Source” describes a magneto-optical trap ion source for a focused ion beam system. U.S. Pat. Pub. No. 2008/0296483 describes a system comprising a magneto-optical trap (MOT), an ionizing laser, and an extraction element. The magneto-optical trap produces a population or “cloud” of supercold neutral atoms. “Supercold” as used herein means cooler than 10 millikelvin. FIG. 1 shows schematically a typical MOT 100. Laser beams 102 slow the neutral atoms and electromagnets 104, which have currents flowing in opposite directions from each other, trap the neutral atoms in a cloud 106.
When the MOT is used as a source of ions in a magneto-optical trap ion source (MOTIS), an ionizing laser ionizes neutral atoms in the trap, and the ions are extracted by an electric field and accelerated in the form of an ion beam toward a target. The cold temperatures in the cloud yield an ion beam with excellent characteristics that theoretically allow for a beam resolution of 10 nm or less. The current produced from this source depends on the operating parameters of the magneto-optical trap and can range from single ions on demand to over 100 pA, a much wider range than is possible using conventional ion sources. In addition, the wide range of elements that can be laser cooled by use of a magneto-optical trap, greatly extends the type and range of ionic species that can be generated and focused into ion beams. FIG. 9 is a periodic table that shows which elements can be used in various types of ion sources.
It is difficult to accelerate ions from a non-point-like source without inducing a large energy spread in the resultant beam. The larger the spatial extent of the ion source, the more difficult it is to focus the ions to a point. Improvements in the system are required to produce smaller probe sizes and produce the resolution that such systems are theoretically capable of producing.